Low-frequency propagation via shallow refracted paths through deep ocean unconsolidated sediments

Abstract

The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. Explosive data from three bottom acoustics stations, located in the Caribbean Sea and Bering Sea, were processed through a high-bandpass (2 to 20 kHz) filter and a low-bandpass (20 to 200 Hz) filter. Bottom returns at the high frequencies were found to be reflections. Bottom returns at the low frequencies were found to come from both reflected and refracted paths. For bottom grazing angles of 90 ° to 25 ° low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25 ° to the angle of intromission (10 °) the dominant contribution of low frequency acoustic energy comes from shallow refracted paths through the sediments. At angles less than the angle of intromission, low-frequency acoustic energy appears to be received from both the refracted and the reflected paths. The refracted path is possible because of the positive velocity gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path should exist wherever unconsolidated sediments exceed a thickness of a few hundred feet. Subject Classification: 30.20, 30.30.